No, he was forced to, after legal action was threatened if he did not. Had Kimmel not receive this letter from TMTG, the video of Kimmel apologizing to Donald Trump from the bottom of his heart and issuing a retraction for lying would not exist otherwise.
It was not a joke. It was a legal demand. Trump forced his hand, and ABC's too, who threatened to fire Kimmel due to his non stop Trump rhetoric.